1. Field of the Invention
The present invention relates to an image suppression filter circuit.
2. Description of the Related Art
Recently, a number of radio terminal systems such cellular phone, PHS, and the like are becoming popular. One of these radio systems is a system for wire communication between base stations, through radio communication between the radio terminal and the base station.
A radio terminal for transmission/reception with the base station through radio wave adopts in general a heterodyne scheme comprising an antenna, a low noise amplifier (LNA), a frequency converter (or mixer), an intermediate frequency band pass filter (IF-BPF), an intermediate frequency mixer (IF-MIX), a low pass filter (LPF), and an analog-to-digital converter (ADC).
The radio terminal, having such a circuit configuration, receives a radio frequency (RF) signal as a high frequency signal by the radio terminal antenna and amplifies it with the low noise amplifier. This amplified high frequency signal is frequency converted from high frequency RF to intermediate frequency by the frequency converter, filtered by the IF-BPF and converted into a digital signal by the A/D converter through the IF-MIX and the LPF.
As an integrated circuit necessary for the radio terminal, there is an image suppression filter circuit for suppressing image signal mixed into a desired frequency.
An image signal is a frequency signal converted into the same frequency band as the intermediate frequency band into which the wanted wave is converted, when the received radio wave is converted from high frequency to intermediate frequency by the frequency converter.
The frequency converter outputs intermediate frequency which is obtained by subtracting a local frequency from a wanted frequency. However, this frequency converter also converts a frequency component obtained by subtracting this intermediate frequency band from the local signal into the same intermediate frequency band. This frequency component is an image wave to become an unwanted wave.
In addition, various systems transmit and receive signals of various frequencies, and a wanted wave of one system becomes an interference wave of another system, and it further becomes an image signal.
Besides, the broadband noise emitted by the transistor itself provides an image signal. Broad band noise includes thermal noise, shot noise or the like.
Such an image signal results in overlapping the same frequency band as the frequency converted wanted band. The waves other than wanted wave are unwanted; however, as the image signal is converted into the same frequency as the wanted wave, an image suppression filter circuit for suppressing image signal is required.
In an image suppression filter circuit used for the frequency converter of the aforementioned radio terminal reception system, first an RF signal is divided into two. One RF signal is frequency converted into an inphase signal by a cosine wave local signal generated in a first 90 degree phase shifter connected to a local oscillator signal, while the other RF signal is frequency converted into a quadrature signal by a sine wave local signal generated in the first 90 degree phase shifter.
Next, the quadrature signal frequency converted by the sine wave is further phase shifted by 90 degrees by a second 90 degree phase shifter, added with the inphase signal which is frequency converted by a cosine wave by an adder, thereby suppressing the image wave.
Incidentally, if frequency conversion is performed simply by the local signal without removing image signal, the signal is folded, and the wanted signal and the image signal are converted into the same frequency. In short, the wanted signal is spoiled by the image signal. Then, if image suppression is performed, the image wave can be reduced while maintaining the wanted wave.
In other words, suppose the wanted wave signal conversion gain be 1, the image wave conversion gain may be reduced to a small number not more than 1 (for instance, 0.01). This can prevent the wanted wave signal from being spoiled by the image wave.
This image suppression filter circuit allows a satisfactory filter function to be achieved to some extent, even when the quality of an inductor or capacitor is low due to integration.
Such an image suppression filter circuit uses a high phase accuracy phase shifter wherein a phase is high and precisely constant for a wide range, or a high amplitude accuracy phase shifter wherein the output amplitude accuracy is high and precisely constant for a wide range image. Though satisfactory in phase accuracy according to an applied frequency, the high phase accuracy phase shifter can not obtain sufficient filter characteristics in a radio system used for broadband, as its output amplitude is not constant.
The high amplitude phase shifter has a goof output amplitude accuracy corresponding to the applied frequency. However, since the output phase accuracy is constant, a sufficient filter characteristic is not obtained for a radio system used in a broadband range.
As mentioned above, a high phase accuracy phase shifter and high amplitude accuracy phase shifter can not obtain high phase accuracy and high amplitude accuracy in the broadband simultaneously, and an image suppression filter circuit used for broadband radio system can not be manufactured on the IC.
In addition, in a conventional portable radio system having a narrow wanted wave band, it was enough that the 90 degree phase shifter had a high phase accuracy and high amplitude accuracy only in a predetermined narrow band. However, in the future, the amount of radio system information is expected to increase, making the wanted wave band broader, and the 90 degree phase shifter will be required to maintain a high accuracy also in the broadband.
However, in the high phase accuracy phase shifter and high amplitude accuracy phase shifter, output amplitude and output phase vary respectively according to the frequency, leading to a problem that the image suppression ratio can not be increased in a broadband radio system.
It is an object of the present invention to provide an integrated image suppression filter circuit that can also be used for a broadband radio system.